This invention relates to improvements in the process for the diffusion welding of copper and stainless steel having corrosion resistance, and more specifically to means for providing a copper-stainless steel clad plates of practical value by use of an insert metal.
Generally, parts of chemical apparatus often use copper-stainless steel clad plates, and those plates are made typically by explosive bonding and roll cladding techniques.
As for means for obtaining thin metal coatings, usually electrolytic plating, hot dipped plating, metal spraying, vacuum vapor deposited coating, and other similar methods are known. However, stainless steels, which are excellently corrosion-resistant alloy steels, cannot be melted to a suitable plating bath and thus fail to lend themselves easily to electrolytic plating. Moreover, because of their higher melting points than copper, stainless steels cannot be used in hot dipped plating. Difficulties are also involved in metal spraying and vacuum vapor deposited coating with the steels.
Diffusion welding is a process in which the objects to be joined together are heated to a temperature approximately above their recrystallization points and are bonded in a solid phase with the application of a pressure. In joining copper and stainless steel, diffusion welding is not in practical use yet because of the inability of attaining adequate bonding strength due, for example, to (1) formation of a hardened, brittle compound layer, (2) "brass cracking" as a result of the diffusion of copper into grain boundaries of stainless steel, and (3) separation and buildup of impurity oxygen out of copper into the interface of bond (which can result in a gas pressure high enough to force the joined surfaces apart), as will be described in the examples to be given later.
Table 1 summarizes the results of experiments in which copper and stainless steel were bonded by ordinary diffusion welding. In the process, copper (tough pitch copper) and stainless steel (SUS 304) were joined in a vacuum atmosphere of 10.sup.-4 torr under varied conditions.
Table 1 ______________________________________ Exp. Welding Welding Bonding No. temp. Pressure time strength* ______________________________________ 1 400.degree. C. 0.1-5 kg/mm.sup.2 1-120 min. 1-2 kg/mm.sup.2 2 800 " " 3-4 3 900 " " 3-5 4 950 " " 3-6 5 &gt;1000 " " 5-7 serious deform. ______________________________________ *in conformity with the ASTM shearing strength test procedure.
As can be seen from Table 1, the direct diffusion welding of copper and stainless steel will produce a bonding strength, or a shearing strength as determined by the ASTM procedure, of at most about 7 kg/mm.sup.2. The shearing strength (8.4 kg/mm.sup.2) required of the clad plate by the ASTM standards is never attained.
As stated, diffusion welding is a process in which the pieces to be joined together are heated upwards of their recrystallization points, and welding is allowed to proceed with the application of a pressure from a fraction to several kilograms per square millimeter. Because the recrystallization point of copper is about 220.degree. C. and that of stainless steel about 400.degree. C., the test data given in Table 1 may be construed to have covered substantially the whole ranges of practical conditions for diffusion welding of copper and stainless steel. The results indicate that at low welding temperatures adequate atomic diffusion does not occur and the resulting weld lacks strength, and at high welding temperatures the abovementioned phenomena (1) to (3) take place and again the weld cannot have satisfactory strength.
FIG. 1 is a microphotograph of the diffusion-welded joint formed (at a welding temperature of 950.degree. C. and at a pressure of 0.5 kg/mm.sup.2 applied for 60 minutes) in Experiment No. 5 of Table 1. In the photograph the symbol .alpha. indicates copper: .beta., stainless steel: .gamma., an alloy layer; and 1, solder brittleness that resulted from the invasion of copper into the grain boundaries of stainless steel.
FIG. 2 is a graphic representation of the hardness distribution (H.sub.v) in the diffusion-welded joint shown in FIG. 1. Throughout the two figures like symbols are used to designate like portions of the joint.
FIG. 3 graphically illustrates the separation of impurity oxygen from copper in the diffusion-welded joint made (at a welding temperature of 800.degree. C. and a pressure of 1.0 kg/mm.sup.2 applied for 60 minutes) in Experiment 2 of Table 1. This figure again uses the symbols in common with FIG. 1.
The present invention has for its object to provide a practical process for the diffusion welding of copper and stainless steel, which overcomes all of the aforedescribed disadvantages of the conventional processes by interposing a suitable insert metal between the surfaces of the copper and stainless steel to be united.
The use of an insert metal is itself a practice not uncommon in the art of diffusion welding. The technique is known, for example, from U.S. Pat. No. 3,530,568 to William A. Owczarski, et al. (issued Sept. 29, 1970). The U.S. patent teaches that, in diffusion welding of Ni alloys containing not less than 6% Al and Ti, a Ni alloy with a 50% or less Co content is employed as an insert metal. Generally, however, the type of suitable insert metal should vary with different materials and different combinations of dissimilar materials to be welded. For example, the insert metal disclosed by the U.S. patent is good only for the particular combination of the particular materials. The present invention is directed to an insert metal for the diffusion welding of stainless steel and copper and also for the conditions for the welding operation, which are both novel without any precedent in the art.